border-1px($color)
  position : relative
    &after
      display : block
      position : absolute
      left : 0
      bottom : 0
      width : 100%
      border-top:1px solid $color
      content : ' '

border-none()
    &:after
      display: none
bg-image($url)
   background-image: url($url+"@2x.png")
  @media (-webkit-min-device-pixel-ratio: 3),(min-device-pixel-ratio:3)
    background-image: url($url+"@3x.png")
